This invention relates to microelectromechanical systems (MEMs), and more particularly relates to electrostatically-actuated structures for MEMs.
MEMs are increasingly being employed for a wide range of applications, in part due to the ability to batch fabricate such microscale systems with a variety of highly complex features and functions. Microscale sensing and actuation applications are particularly well-addressed by MEMs. For many MEMs applications, electrostatically-actuated structures are particularly effective as analog positioning and tuning components within complex microsystems. Electrostatic actuation provides a combination of advantages for the microscale size regime of MEMs, including the ability to produce high energy densities and large force generation, as well as the general ease of electrostatic actuator fabrication, and high operational speed due to relatively small mass. Indeed, for many MEMs applications, electrostatic actuation is preferred.
Electrostatic actuation of a structure is typically accomplished by applying a voltage between an electrode on the structure and an electrode separated from the structure. The resulting attractive electrostatic force between the electrodes enables actuation of the structure toward the separated electrode. This applied electrostatic force is opposed by a characteristic mechanical restoring force that is a function of the structure""s geometric and materials properties. Control of the structure""s position during actuation requires balancing the applied electrostatic force and inherent mechanical restoring force.
The electrostatic force is a nonlinear function of distance; as the structure moves toward the separated electrode, such that the electrodes"" separation distance decreases, the electrostatic force between the electrodes typically increases superlinearly. In contrast, the mechanical restoring force of the structure typically is a linear function of distance. Due to this disparate dependence on distance, not all positions between the electrodes are stable. Specifically, at electrode separations less than some minimum stable separation characteristic of the structure, the structure position is unstable and causes uncontrollable travel of the structure through the remaining distance to the separated electrode. This instability condition, known generally as xe2x80x9cpull-in,xe2x80x9d is a fundamental phenomenon resulting from the interaction of the nonlinear electrostatic force with the linear, elastic restoring force of the structure being actuated. Generally characteristic of all electrostatically-actuated structures, pull-in instability is well-known to severely limit the fraction of an electrode separation gap through which such a structure can be stably positioned.
A separate but related limitation of electrostatically-actuated structures is the relatively high voltage level typically required to position such a structure through a relatively large stable actuation range. As a result of this limitation, in combination with the electrostatic pull-in limitation, electrostatically-actuated structures typically are not well-suited to produce a large range of actuation motion. But for many microscale positioning and tuning systems, such as optical systems, large ranges of travel, and analog tuning of position, can both be critically required. There thus remains a need for electrostatic actuation techniques that enable large ranges of travel and further that are optimized for actuator operation at the lowest possible operational voltage.
The invention provides electrostatic actuator configurations and actuation mechanisms that enable stable, large range-of-motion electrostatic actuation, achievable with relatively low actuation voltages.
An example electrostatically-controllable actuator in accordance with the invention is an electrostatically-controllable diffraction grating. The diffraction grating includes a plurality of electrically isolated and stationary electrodes that are disposed on a substrate. At least one row of a plurality of interconnected actuation elements is provided. Each actuation element in a row is suspended, by a corresponding mechanically constrained support region, over the substrate by a vertical actuation gap. Each actuation element includes a conducting actuation region connected to the corresponding support region and disposed in a selected correspondence with at least one substrate electrode.
A mirror element is provided for at least one actuation element in at least one row of actuation elements. Each mirror element includes an optically reflecting upper surface. Each mirror element is vertically suspended over a corresponding actuation element by a mechanically constrained mirror support region that is connected to the corresponding actuation element and that defines a vertical mirror gap. Each mirror element also includes a mirror deflection region connected to the mirror support region and free to be deflected through the mirror gap.
The mirror gap of a mirror element is provided as less than the actuation gap of a corresponding actuation element. With this consideration, the mirror gap is then selected to produce controlled and stable displacement of the actuation region of a corresponding actuation element through a displacement range to a specified point in the actuation gap when an actuation voltage is applied between an actuation region and a corresponding stationary electrode. This enables the diffraction of a beam of light incident on the grating as the beam of light is reflected from the upper surfaces of the mirror elements.
The mirror element can be provided as a row of a plurality of interconnected mirror elements provided for at least one row of actuation elements. A plurality of rows of actuation elements can be employed, here preferably with a mirror element provided for at least one actuation element in at least one actuation element row or at least one of the actuation element rows being provided with a row of interconnected mirror elements.
In one scenario, the mirror gap can be further selected to maintain a substantial planarity of the mirror element deflection region during stable displacement of a corresponding actuation region. The mirror gap selection further can be made to maintain a substantial parallelism of a planar mirror element deflection region with the substrate during the corresponding actuation region""s stable displacement.
Preferably, the optically reflecting upper surface of at least one mirror element in a row of mirror elements is characterized by a substantial planarity as a corresponding mirror deflection region is deflected through the mirror gap. This planar, optically reflecting upper surface can for some applications also preferably be maintained parallel with the substrate as a corresponding mirror deflection region is deflected through the mirror gap; additionally or alternatively, the planar optically reflecting upper surface can be maintained parallel with that of at least one other mirror element provided for another row of actuation elements, and can be provided as parallel with the optically reflecting upper surface of all other mirror elements.
For each actuation element there can be defined an actuation element deflection region that is free to be deflected through the actuation gap. Here the actuation region extends from about the actuation support region to the actuation element deflection region. A commonality in area between the actuation region and a corresponding stationary electrode is preferably here selected to produce controlled and stable displacement of the actuation element deflection region over a displacement range extending to the specified point in the actuation gap.
The substrate of the diffraction grating can be provided as, e.g., silicon, or other suitable material; an insulating surface layer can be provided on the substrate. The electrodes, as well as the actuation elements and the mirror elements, can all be provided as polycrystalline silicon. The optically reflecting upper surface of the mirror elements can be provided as a gold layer.
The diffraction grating can be employed for a wide range of applications requiring the ability to diffract an incoming beam of light, particularly optical applications. The elements of the grating outlined above define an electrostatically-controllable actuator that can in one embodiment be generally described as having a stationary electrode and an actuation element that is separated from the stationary electrode by an actuation gap. The actuation element includes a mechanically constrained actuation support region and a conducting actuation region connected to the actuation support region and free to be deflected through the actuation gap.
An auxiliary element is provided, separated from the actuation element by an auxiliary gap. The auxiliary element includes a mechanically constrained auxiliary support region that is connected to the actuation element, and further includes a deflection region that is connected to the auxiliary support region and is free to be deflected through the auxiliary gap. The auxiliary gap is selected to be less than the actuation gap. With this consideration, the auxiliary gap is selected to produce controlled and stable displacement of the actuation region over a displacement range extending to a specified point in the actuation gap when an actuation voltage is applied between the actuation region and the stationary electrode. The auxiliary gap is further selected to maintain a substantial planarity and parallelism of the auxiliary element deflection region with the stationary electrode during stable displacement of the actuation region. Preferably the auxiliary element includes a horizontal upper surface that is characterized by a substantial planarity that is maintained as the actuation region is displaced.
The actuation element can be defined to include an actuation element deflection region that is free to be deflected through the actuation gap. Here the actuation region extends from about the actuation support region to the actuation element deflection region. A commonality in area between the actuation region and the stationary electrode is selected to produce controlled and stable displacement of the actuation element deflection region over a displacement range extending to the specified point in the actuation gap.